1. Field of the Invention
This invention relates to a three-dimensional shape measurement apparatus which directs a laser beam at an object, picks up the light reflected back from the object, photoelectrically converts the light and subjects it to signal processing to obtain three-dimensional information about the object.
2. Description of the Prior Art
There are many non-contact, optical methods of measuring the three-dimensional shape or profile of an object ( See, for example, Applied Optics, Vol.21(1982) page 3200, Vol. 23(1984) page 3837 or Vol. 25(1986) page 1630). One medical application of an optical three-dimensional measuring method that has been attracting attention recently is an apparatus for obtaining three-dimensional information about the human eye fundus.
As well as its use in ophthalmology, examination of the eye fundus is also employed in internal medicine for diagnosing hypertension, diabetes, diseases of the cerebral nerves and other such disorders. For this, one method in widespread use is that of using a fundus camera to conduct a photographic examination. However, because quantitative measurement, particularly of the degree of depression of the optic papilla of the eye fundus is useful in the early detection of glaucoma, and as such is directly related to the prevention of loss of vision, recently there have also been attempts to acquire three-dimensional information about the fundus, in addition to the two-dimensional information provided by the usual fundus camera.
One such three-dimensional fundus measuring method consists of projecting a fixed stripe or grid pattern on the fundus and observing it at a prescribed angle from a separate direction in order to measure the deviation of the stripe or grid image from a straight line. The principle of triangulation is then used to convert the amount of deviation in the depth direction and thereby quantitatively evaluate the state of fundus depression (see, for example, U.S. Pat. No. 4,423,931).
In another method, based on the principle of stereoscopic photography, a fundus camera or the like is used to take two photographs of the fundus at different angles of pupil incidence. By then analyzing the images of the two fundus photographs, depth information can be extracted and quantified. Apparatuses have also been developed in which the photographic film of the fundus camera is replaced by two equivalent television cameras positioned at different observation angles and linked to a computer so as to provide three-dimensional information automatically (see, for example, U.S. Pat. No. 4,715,703).
However, with each of these methods still suffering from low spatial resolution capabilities and difficulties relating to accuracy and the degree of reproducibility, their clinical feasibility remains uncertain. One reason for this relates to physical restrictions inherent in the fundus of the human eye; i.e., observation of the inner surface of a relatively large spherical object via the limited window of a small pupil. In the case of both triangulation and stereophotography, it is not possible to increase the angular difference of the observations. Another reason is the very low reflectivity and contrast of the fundus, which is a product of reflection characteristics specific to the object, i.e., the abrupt changes in the reflection intensity of the optic papilla of the eye fundus. It is mainly for these two reasons that, in the case of measuring three-dimensional shapes, it is difficult to improve spatial resolution capabilities, accuracy and reproducibility.
Another problem with methods of measurement using the conventional apparatuses is that none of them are methods of directly measuring depth information, and as such, in each case processing time is required. The method employing the principle of triangulation requires time for the calculations involved in the conversion of the amount of stripe or grid image deviation to depth information, and the stereophotographic-based method requires time for analyzing the image information in the two photographs to elucidate depth information. Also, there is a tradeoff between shortening of the processing time and improvement in the spatial resolution. Computer advances can reduce the time needed for the processing, but not enough for clinical applications.
One electronic ophthalmic examination apparatus for human eye fundus applications that employs laser scanning was developed by the Retina Foundation of the U.S.(See U.S. Pat. No. 4,213,678 and Japanese Laid-open Patent Publication 62-117524.) This apparatus has attracted attention for the many features it possesses, such as the ability to display in real-time on a monitor television a high-contrast video image of the eye fundus, using a low level of illumination. In a paper (Applied Optics, vol. 19 (1980) page 2991), the Retina Foundation mentioned that it was feasible to use this laser-beam scanning method to ascertain the three-dimensional configuration of the eye fundus. However, the method is based on the same principle of stereoscopic photography explained above, in which different angles of pupil incidence are used, so there are problems concerning resolution and accuracy. As such, regarding the direct determination of the three-dimensional shape of the fundus, the method is not practicable.